docs: def _build_ranklist(self, phrase_list):
          """Method to rank each contender phrase using the formula
  
                phrase_score = sum of scores of words in the phrase.
                word_score = d(w)/f(w) where d is degree and f is frequency.
  
          :param phrase_list: List of List of strings where each sublist is a
                              collection of words which form a contender phrase.
          """
          self.rank_list = []
          for phrase in phrase_list:
              rank = 0.0
              for word in phrase:
                  if self.metric == Metric.DEGREE_TO_FREQUENCY_RATIO:
                      rank += 1.0 * self.degree[word] / self.frequency_dist[word]
                  elif self.metric == Metric.WORD_DEGREE:
                      rank += 1.0 * self.degree[word]
                  else:
                      rank += 1.0 * self.frequency_dist[word]
              self.rank_list.append((rank, " ".join(phrase)))
          self.rank_list.sort(reverse=True)
          self.ranked_phrases = [ph[1] for ph in self.rank_list]